limitations of the manage tab (3 posts)

I have twelve menu items on my manage tab five standard pages, posts etc and five from plugins. The problem is there should be some additional plugin menu items but only the twelve mentioned are displayed at any time. No second row gets created as in the options tab. If I deactivate one of the plugins currently displayed others (that should be there) show up.

Is there a limitation to the number of menu items on the manage tab? Is there a way around this?

I don't know if this a bug or a problem with the way the plugins are written but I have verified this now with three different plugins Now reading, AMM, and category visibility, they pop in and out of the manage tab menu. activate one and another disappears.

I'm sure it has something to do with the way the menu are being created in the plugin
ex (add_submenu_page('edit.php', 'Category Visibility', 'Category Visibility', 10, basename(__FILE__), 'category_visibility'); )